The seismicity of the Pacific coast of Tohoku, Japan has been investigated in detail and characterized into regional seismic segments. The 2011 megathrust earthquake of Mw9.0 on 11 March ruptured almost all of the segments in that area, causing devastating tsunami. The prime factor that had not been recognized before is the double segmentation along the Japan trench: The apparent absence of earthquakes in the trench-ward segments as opposed to the Japan Island-ward segments that have repeated smaller earthquakes. We term this pattern of seismic activity along-dip double segmentation (ADDS). The 2011 Tohoku megathrust is typical of a class of great earthquakes different from that of the 1960 Chile, in which a young and buoyant plate is subducting rapidly under the continental plate. In the 1960 Chile case, the seismic activity is characterized by along-strike single segmentation (ASSS), where there is weak seismic activity before the main event all over the plate interface of the subduction zone. We study the greatest earthquakes around the world and find that there is a variety of megathrust earthquakes characterized by ASSS to ADDS, where the 2004 Sumatra- Andaman, the 1960 Chile, the 1964 Alaska and the 2011 Tohoku-oki earthquakes are typical end-members.